The creation of particulate matter is involved in a number of processes, including atomization for paint spraying, metal powder atomizing, fuel injection, and the production of pharmaceutical compositions. For many of these processes, the size and consistency of the particulate matter is central to the use. For example, the relationship between particle size and rate of release of a pharmaceutical composition is an important issue for pharmaceutical companies, since drug absorption (e.g. into the lungs) is controlled in large part by the size of the drug. It is thus critical for devices for pulmonary delivery of drug to deliver a controlled amount of drug that is available in the correct size range for proper adsorption and activity. If the pharmaceutical is outside the optimum range for proper pulmonary delivery it is wasteful of the material and, more importantly, the dosage of the pharmaceutical received by the patient cannot be controlled.
Particulate matter can be formed using any of a number of processes, including atomization. Atomization generally utilizes two fluids, a first containing the matter to be atomized, and a second fluid which functions to physically break up the subject fluid into droplets or particles. Existing atomization methods convert the type of energy supplied to the system into surface tension free energy since the fluid interface is dramatically expanded by the effect of the applied energy. Thus, the kinetic energy of the gas in pneumatic atomizers, the electrical energy in sonic and ultrasonic piezoelectric atomizers, the mechanical energy in rotary atomizers, and the electrostatic energy in electrohydrodynamic atomizers directly impact on the rate and efficiency of the formation of particles. As a function of the resulting degree of disorder, part of the energy is also degraded in the statistical dispersion of the resulting drop sizes. Depending on how disorderly and rapidly (or gradually and efficiently) the processes by which the above-mentioned energies are converted into free surface energy take place, the resulting sprays are suitable for different specific uses.
Existing pneumatic atomizers involve the cascading breakage of the interface from a high Weber number to a unity Weber number, the unity Weber number being attained when drop diameters result in surface tension forces that offset the inertia of the gas relative to the liquid. Such atomizers are described in S. Nukiyama and Y. Tanasawa, Trans. Soc. Mech. Eng. Jpn., 5, 68-75, (1939); I. D. Wigg, J. Inst. Fuel, 27, 500-505 (1964), G. E. Lorenzetto and A. H. Lefebvre, AIAA J., 15, 1006-1010 (1977); A. K. Jasuja, ASME Paper 82-GT-32. (1982); N. K. Risk and A. H. Lefebvre, Trans. ASME J. Eng. Gas Turbines Power, 106, 639-644, (1983); and A. Unal, Metall. Trans. B., 20B, 613-622 (1989). Cascading processes in existing pneumatic atomizers involve highly turbulent flows and randomness, which can result in highly disperse drop size and atomizates. In addition, pneumatic atomizers are limited in the size drops they can provide (above 20 microns on average at best).
Whistling atomizers also have their pitfalls, prominent among which are noise, a relative complexity stemming from the use of wave generators and piezoelectric devices to excite the capillary jet produced, and a limited drop size that is generally larger than 50 .mu.m.
One novel atomization system that can provide smaller, monodisperse drop sizes is electrostatic or electrospray atomization. The system has been disclosed by M. L. Colelough and T. J. Noakes, European Patent Application 87305187.4 (1987). The chief disadvantage of this method is that the energy required to create the atomizate requires using a high-voltage DC source, and hence a discharge system (e.g. electrical crowns), both of which add up to inherent complexity, large weight and low manipulability in this system.
Combined withdrawal of an interface between two immiscible fluids (e.g. two immiscible liquids or a liquid and a gas) has recently begun to be studied. See e.g. E. O. Tuck and J. M. van den Broek, J. Austral. Math. Soc. Ser. B., 25, 433-450, (1984); L. K. Forbes and G. C. Hocking, J. Austral. Math. Soc. Ser. B., 32, 231-249, (1990); and T. J. Singler and J. F. Geer Singler, Phys. Fluids A, 5, 1156-1166, (1993). The onset of combined withdrawal results in the sweeping of the fluid behind the free surface when the fluid in front of it is withdrawn at a given distance from the surface. Studies in this field have focused largely on the determination of parameters such as the distance from the sink to the free surface, the fluid density ratio, and the surface tension between the fluids, at the onset of combined withdrawal. However, the fluid dynamics of the microjet produced by combined withdrawal remained unexplored.
There is a need in the field for a method of reliably and reproducibly producing particles within a desired size range without expending a great amount of energy. Moreover, there is a need for such a method that produces a smaller and more uniform particle size.